The present invention relates to a radio receiver, for example, to a microwave band radio receiver for receiving signals derived from broadcast waves which have been radio-transmitted in the microwave band.
Generally, a radio receiver includes a mixer which generates an intermediate frequency signal (hereinafter, referred to as IF signal) from a received radio modulation signal (hereinafter, referred to as RF signal) and a local oscillation signal (hereinafter, referred to as LO signal) generated in the receiver.
As for electric power of an LO signal to be inputted to the mixer and operating bias of the mixer, appropriate values points are chosen so that successful IF signal can be obtained by demodulation in terms of conversion gain, noise characteristics, low distortion or the like. The electric power of the LO signal is constant and, generally, large enough as compared with the electric power of the RF signal. Accordingly, the operating bias current of the mixer has a value dependent on the power of the LO signal, and hardly not on the power of the received RF signal, thus being a generally constant value.
In recent years, there have been energetically developed radio receivers using frequency bands of 10 GHz or higher microwave and milli-wave bands, and such radio receiver and radio reception method as shown below have been proposed (see, e.g., JP 2003-258655 A).
FIG. 10 shows the construction and frequency arrangement of a proposed radio receiver 900. The radio receiver 900 receives a radio multiplexed signal 930 transmitted from transmitter with an antenna 901. This radio multiplexed signal is formed from combination of a first local oscillation signal 930c (frequency: fLO1) and a first radio modulation signal 930a. This first radio modulation signal 930a, in which a first local oscillation signal 930c and a first intermediate frequency signal 932a (IF1 signal, frequency: fIF1) are multiplied, has a frequency of fLO1+fIF1.
The received radio multiplexed signal 930 has its unwanted waves eliminated by a filter 902, and amplified by an amplifier 903. Then, with the use of a mixer 905, the amplified radio signal and a second local oscillation signal (frequency: fLO2) generated from a local oscillator 904 inside the receiver are multiplied and down-converted, by which a first-local-oscillation-signal component signal 931c (frequency: fLO1−fLO2) and a first-radio-modulation-signal component signal 931a (frequency: fLO1+fIF1−fLO2) are generated.
Next, the signals, after the amplification by an amplifier 906, are divided by a divider 907, one being amplified by an amplifier 910 via a filter 908 allowing only the first-local-oscillation-signal component signal 931c and then inputted to a mixer 911, and the other being inputted to the mixer 911 via a filter 909 that allows only the first-radio-modulation-signal component signal 931a to pass through.
In the mixer 911, the first-local-oscillation-signal component signal 931c and the first-radio-modulation-signal component signal 931a are multiplied and down-converted, by which the IF1 signal 932a is demodulated. This is expressed by the following equation:(fLO1+fIF1−fLO2)−(fLO1−fLO2)=fIF1
In this method, it is described that since frequency fluctuations and phase noise of the first local oscillation signal and the second local oscillation signal are canceled during the demodulation of the IF1 signal, there is no need for any high-performance oscillator so that the manufacturing cost can be reduced. Further, it is also described that since the first-local-oscillation-signal component signal 931c is amplified by the amplifier 910, a signal of sufficiently high level as an LO power can be fed to the mixer 911, so that the demodulation sensitivity can be enhanced.
However, with the above-described radio system, the LO power to be inputted to the mixer changes in proportion to the received power of the radio multiplexed signal 930. The LO power is small when the received power is small, while conversely the LO power is large when the received power is large. Therefore, with the use of a conventional mixer, the operating bias current of the mixer would largely change due to the received power of the radio multiplexed signal 930. As a result, in the case of transmission of digital terrestrial broadcasting or BS/CS television broadcasting waves, when the transmission distance is shortened so that the received power is enlarged, there would be some cases where the operating state of the mixer (operating bias) is changed, causing the reception C/N (carrier to noise ratio) to deteriorate or the mixer to be broken due to occurrence of a large current.
Moreover, when the operating bias of the mixer is so set as to prevent occurrence of the such troubles as described above even with a large received power, it would occur that the mixer does not operate when the received power is small, resulting in a small output of the mixer. In this case, the reception C/N could not be ensured and the transmission distance could not be elongated.
Consequently, with such a radio system, the dynamic range of the mixer would be narrowed so that stable reception C/N characteristics or image characteristics could not be obtained over a wide range of transmission distance in the radio system.